Hybrid vehicles with high efficiency have recently become widespread in response to a growing concern about global environmental conservation. The efficiency of a hybrid vehicle is increased by storing energy in an electricity storage unit and using the stored energy as necessary. A bidirectional DC-DC converter is used to control the charging and discharging of the electricity storage unit.
In conventional bidirectional DC-DC converters, hard switching is dominant in which a switchover is made while a voltage is applied to a switching device or current is flowing in it. The action by which a switching device is turned on from the turned-off state or turned off from the turned-on state is called switching. The efficiency of hard switching is low because the energy loss involved in switching is large.
A resonant bidirectional DC-DC converter that reduces the loss involved in switching to increase the efficiency is disclosed in Japanese Patent Laid-open No. 2003-33013 (Patent Document 1). This resonant bidirectional DC-DC converter performs boost and buck operations between 12-V and 42-V power supplies, so a pair of main metal oxide semiconductor field effect transistors (MOSFETs) and a main smoothing reactor are used as a basic main circuit. In addition to this basic main circuit, a pair of auxiliary MOSFETs and a resonant reactor are used as an auxiliary resonant circuit so as to implement soft switching. The pair of main MOSFETs are connected in series between both buses of a second power supply. An auxiliary smoothing reactor is connected between the series connection points and the positive bus of a first power supply. A snubber (buffer) capacitor is connected in parallel to each of the pair of main MOSFETs. The auxiliary resonant circuit is connected between both ends of the main reactor; the auxiliary resonant circuit comprises a pair of anti-series auxiliary MOSFETs and a series auxiliary (resonant) reactor.
The auxiliary resonant circuit controls the switching of the auxiliary MOSFET pair in both the boost and buck operations so that the charging and discharging of the snubber capacitors connected to the main MOSFETs are controlled by resonant current flowing in the resonant reactor, thereby achieving soft switching.
A resonant bidirectional DC-DC converter is disclosed in Japanese Patent Laid-open No. 2005-176540 (Patent Document 2), in which four main transistors are connected in a bridge form, a main rector is connected among their bridging points, and an auxiliary resonant circuit is connected in parallel to the main reactor. This auxiliary resonant circuit also comprises a pair of anti-series auxiliary transistors and a series auxiliary (resonant) reactor as in (Patent Document 1).
In Japanese Patent Laid-open No. 2005-184965 (Patent Document 3), a bidirectional DC-DC converter using many devices is disclosed.
Japanese Patent Laid-open No. 2004-343923 (Patent Documents 4), 2001-37214 (Patent Documents 5), 2003-153527 (Patent Documents 6), 2003-189602 (Patent Documents 7), and 2004-201373 (Patent Documents 8) disclose technologies for increasing the efficiency of a unidirectional DC-DC converter by reducing a switching loss.